This invention relates generally to two-dimensional light valve display devices, and more specifically to such devices of the type using a liquid crystal material or the like.
A great deal of emphasis has been placed in recent years on liquid crystal flat panel display devices. Recognized advantages of such devices are low electrical power consumption and small size. Liquid crystal displays are widely used in watches and other small battery driven consumer products. In recent years, larger liquid crystal display devices have been developed and utilized in portable computers and the like. Such general display devices individually control separate pixels or dots across the liquid crystal layer in order to form virtually any type of alpha-numeric character set or general graphics images. The size of each individually controlled pixel or dot determines the smallest resolution element size of a displayed image.
The liquid crystal material most commonly utilized for such displays is normally in an optically neutral state in the absence of any significant electrostatic field applied through the liquid crystal layer. When an electrostatic field above some threshold is applied therethrough, the material rotates the polarization of light incident upon it and also changes its index of refraction, so long as the field remains. When the electrostatic field drops below the threshold, the material returns to its neutral state. Thus, the application of an electrostatic field above the threshold in a particular pattern across the two-dimensional display will cause the portions of liquid crystal so exposed to change its visible optical characteristics with respect to other portions, thus controllably displaying a pattern. If the effect of the liquid crystal material on light polarization is being utilized, the pattern is viewed through an optical system that includes polarizers. If the change of index of refraction characteristic is being utilized, it is viewed through a Schlieren optical system.
Such devices generally apply the electrostatic field through the liquid crystal layer by electrode surfaces on either side thereof. A voltage is not necessarily applied to these electrodes for the entire duration that the display is desired, but rather voltage thereacross is periodically applied, an electrostatic field being maintained between voltage pulses by inherent capacitance of the device. If subsequent display refreshing pulses are not applied, this capacitance will discharge in a short period of time and cause the liquid crystal to return to its neutral state. Such liquid crystal material also has the characteristic of responding unfavorably to a direct current electrostatic field, so the refreshing pulses are commonly made to have an alternating polarity which maintains the average electrostatic field at zero.
Another type of liquid crystal, which is not as commonly used, selectively changes the relative phase of light incident upon it and has a "memory"; that is, the optical change of material in response to a single application of an electrostatic field through it stays that way indefinitely. In order to make the material clear again, therefore, an erasing electrostatic field must be applied. This type of material is characterized by scattering incident light in regions where the field is applied, when it is used with an adjacent glass layer that is appropriately treated.
It is a primary object of the present invention to provide a light valve device having a liquid crystal or similar material that changes optical characteristics in response to electrostatic fields, and driving and controlling circuits provided integrally therewith.
It is another object of the present invention to provide such a device that is economical to manufacture, simple to incorporate into electronic systems and which is capable of changing its image at a high rate of speed.
It is a further object of the present invention to provide a liquid crystal display device of small size but which is capable of providing a high resolution display that may be magnified before viewing by projection optics or otherwise.